The invention relates generally to the field of power generation units used for power generation for utility grids, and more particularly to techniques for ensuring grid compliance of power generation units, including stabilizing power during transient conditions.
An increasing number of power generation units such as wind turbines, solar cells, fuel cells, wave power systems or the like are connected to the utility grid. The need for frequency support for the utility grid becomes greater as the conventional power plants comprising large synchronous generators are replaced by power generation units such as wind turbines. The power generation units are connected to the utility grid using power electronics, and the direct link between power and frequency is lost, whereby the disturbances on the grid might result in larger frequency deviations. The challenge for a wind turbine is used in the following in order to describe the background of the invention. However, the challenges are similar for other types of power generation units such as solar cell, fuel cell, micro turbine, wave power or the like where the interface between the utility grid and the power generation unit is based on a power converter.
A wind turbine generator generally includes a wind rotor that converts wind energy into rotational motion of a turbine shaft, which in turn drives the rotor of an electrical generator to produce electrical power. Modern wind turbine generator installations typically take the form of a wind farm having multiple wind turbine generators connected to a common wind farm power grid. This wind farm grid is connected to a utility grid, either directly or through a substation which may include a step-up transformer.
Individual wind turbines and wind farms are required to comply with the power quality requirements of the utility system operator. Such power quality requirements, often designated as “grid requirements” may typically include voltage regulation, frequency regulation, active and reactive power control, fault ride-through, and in some cases also power ramping and the provision of spinning reserve or inertia in case of transient conditions caused by sudden failure of generation, line fault or connection of rapid application of large loads.
From a utility point of view it would be preferable if wind turbine generators could be fitted with classical synchronous generators having the same regulation capabilities as the synchronous generators applied at large hydro or thermal power plants. Such classical synchronous generators are capable of regulating voltage, active and reactive power etc. In transient conditions, the synchronous generators may also provide additional control services that modulate active power to stabilize the power system and restore frequency to its nominal value.
However, classical synchronous generators are not well suited for use on wind turbines, since their very stiff characteristics are not compatible with wind turbine application. In order to approximate synchronous generator operation and capabilities modern wind turbine generators typically use power electronic inverters to interface the wind turbine generator output with the utility grid. In one common approach the wind turbine generator output is directly fed to a power electronic converter, where the turbine frequency is rectified and inverted into a fixed frequency as needed by the utility system. An alternative approach uses a doubly fed asynchronous generator (DFAG) with a variable frequency power electronic inverter exciting the DFAG rotor and stator windings being coupled directly to the utility system.
Traditionally, wind turbine generators have been configured to respond to the grid requirements through the use of a combination of grid measurement devices, utility signals, and response references and algorithms internal to the turbine controller.
This arrangement has a number of drawbacks. Firstly, the wind turbine generator response to grid requirements generally becomes a black box seen from the perspective of the system operator. Secondly, feed-back response elements may occur where the wind turbine generator system regulates in response to self-created artifacts. Furthermore, in the normal configuration wind turbines do not contribute to the frequency stabilization of the utility system.
The purpose of the invention is to overcome the above mentioned limitations for power generation units and to provide control techniques so that the power generation unit can meet the grid requirements in a way that is transparent to system operators, including contributing to frequency regulation and power-swing stabilization for the utility system.